The twentieth century opened with the discovery of radio wave transmission by Marconi. World War II heralded the emergence of radar. The 1960's witnessed the launching of satellites. The 1990's saw the proliferation of commercial wireless data communications. These four events signaled epochal moments in history, opening up entirely new ranges of the electromagnetic spectrum for revolutionary applications such as radio, television, long-range surveillance, satellite communications and computer networking. The key components that made these advances possible were the development of electronic components capable of detecting, amplifying and re-transmitting high-frequency electrical signals: the point contact diode, the vacuum tube triode, the semiconductor transistor, the traveling wave tube, the integrated circuit. Each had—or is having—its moment and was superceded by a newer technology as demand for higher performance increased.
Today, RF communications, radar and other applications are pushing well into the high gigahertz region, as much as 200 GHz or more. Even home wireless networking and simple cordless telephones are operating at over 5 GHz, a domain once reserved to only the military a few short decades ago.
The key components that made these advances possible are high-frequency devices: transistors with current-gain-bandwidth product fT>200 GHz, LNAs with high linearity (IIP3), emerging power transistors made of SiC and GaN, and the venerable traveling wave tube (TWT). Many applications such as digital radio and military surveillance today are limited by the power or bandwidth achievable in a conventional semiconductor, or by the size, weight, cost, power and distortion products of the TWT. Space electronics is also limited by the radiation hardness and reliability of semiconductors. Military applications also require greater bandwidth, with tuning ranges exceeding 10:1 at frequencies up to 100 GHz.
Semiconductor Amplifiers
Despite the ubiquity of modem semiconductors, they suffer several limitations for the highest frequency RF applications. First, transistor breakdown voltage must be reduced significantly to achieve the necessary bandwidth, often to a volt or two or less. This severely limits the power they can generate, especially when low distortion is required. More fundamentally, semiconductors have an upper bandwidth dictated by the physics of the semiconductors: the maximum carrier velocity, especially, the saturated electron velocity. Current art places a limitation of perhaps 400 GHz fT on III-V compound devices such in InP, GaAs, InAs, and a theoretical limit of approximately 1 THz is dictated by the velocity of current-conducting carriers (electrons) in any semiconductor crystal. Practical applications such as an RF low-noise amplifier (LNA) usually can only operate at no more than 1/10 of the fT. Furthermore, to operate at speeds of 100 GHz or more (as in an RF LNA) requires considerable power. At this time, there are almost no semiconductor power amplifiers capable of operating much above 10 GHz, leaving the entire field of high-power antennas to the field of vacuum electronic devices, such as the TWT, which are orders of magnitude more expensive and bulky. Semiconductor amplifiers are also extremely sensitive to radiation induced degradation and failure in space environments.
TWTs and Other Traditional Vacuum Electronic Devices
TWT's offer direct RF amplification with power gains exceeding 40 dB, frequency of amplification over 100 GHz, and bandwidth of more than 2 octaves in specialized devices. The drawback is they are large, very expensive, power consumptive, noisy and introduce significant signal distortion. Size can vary from 10 cubic inches in very high frequency devices (˜100 GHz). Cost can be $10,000 in a typical device to as much as $100 k in a space-rated device. Minimum power consumption can be hundreds of watts even in a low power device. Noise figures are typically 40 dB, compared to as little as 1 dB in a semiconductor LNA. Distortion products for wideband operation can be similarly oppressive, restricting their use to power amplification. TWTs can in principle operate at frequencies approaching or exceeding 1 THz, but become extremely inefficient at these frequencies (as little as a few percent), and very hard to build because of the micron-sized dimensions. Machining tolerances of a few nanometers become necessary, and waveguide losses become dominant, since a long waveguide (such as a helix, serpentine, or many coupled cavities) has unavoidable ohmic sidewall losses.
Many applications today are severely constrained by the lack of high-frequency performance in available amplifiers. For example, an emerging application is wireless networking in dense urban environments. The demand for communication bandwidth on network channels is already exceeding 1 Gbps, yet the limits of present-day carrier frequencies is only about 5-10 GHz. As is known in the art, the carrier frequency must normally be much higher than the data rate—100 times higher or more. For example, 2.4 Ghz carriers typically provide 10 Mbps data rates or less in the well-known “Bluetooth” system (sometimes called “802.11b”). 1 Gbps data rates imply a carrier of at least 100 GHz or more.
The problem is exacerbated in dense urban environments, especially around large office buildings. Current technology increases the spectrum capacity by limiting the range of a limited number of sub-channels (which may be spectrally broad in spread spectrum or Ultra Wideband (UWB) systems). No more than a few hundred low-bandwidth (10 Mbps) channels can typically be made available within a short geographic radius of a few hundred meters. In an urban environment with thousands of network connections within a single building and other buildings in close proximity, it can be seen that there is a hard limit, indeed, on the number of network connections and the aggregate data transfer rate that is possible per cubic mile.
Hard-wired networks traditionally overcome this density limitation, but they are difficult to install and very expensive to retrofit an existing structure. Wireless systems have recently proliferated (based on the 802.11b standard, among others) using higher carrier frequencies, but for higher bandwidths and link densities, few or no solutions exist today.
As mentioned, semiconductor amplifiers cannot operate much above 100 GHz with any gain at all, and are very power inefficient. TWT amplifiers also cannot operate efficiently much above 100 GHz (though they are much better), but are prohibitively expensive for most applications. What is needed is a solution that offers the size and economies of scale of semiconductors, and the gain and frequency performance of TWTs, with power efficiency and linearity greater than both. Thus, it can be appreciated that there is a real demand for a low cost, efficient millimeter wave to sub-millimeter wave RF technology.
Related Art
As will become apparent, the present invention relates to microminiature electron beam devices applied to RF amplification and signaling, particularly those that operate in the millimeter to sub-millimeter wave region (50 GHz to 2 THz). Similar inventions have claimed advances that might operate in this region. For example, Manohara et al (ref. 11) have published work on sub-millimeter “nano-klystrons” based on many of the elements described herein for the present invention: semiconductor fabrication, MEMS and electron gun construction. An impressive development, it nonetheless suffers many deficiencies, including narrowband tuning, and relatively slow response to signal modulation, because of the resonant cavities inherent in the method. The nano-klystron also lacks integral phase and polarization control, which are highly desirable features of any RF power device intended for transmission purposes, yet expensive and bulky to provide as separate elements.
U.S. Pat. No. 5,497,053 issued to Tang, et al shows a deflection amplifier (or “deflectron”) that purports to offer wideband amplification, but suffers low gain, relative to the invention here, because the detrimental effects of space charge repulsion limit the maximum beam current. Furthermore, such beam current as Tang et al. can generate creates significant heating losses. Tang et al. also does not offer integral solutions to antenna coupling, phase and polarization control.
U.S. Pat. No. 3,725,803 issued to Yoder predates Tang et al., and teaches an electron beam driven P-N junction in a push-pull detector arrangement. Yoder does not suggest his method provides extra gain through the beam interaction with the semiconductor diodes, though it may be inferred. However, such extra gain as may be provided will be modest, and the apparatus does not lend itself well to microfabrication. Further, Yoder does not adequately elaborate on how his method will provide linear gain, and it may be inferred from the description that high linearity will not be achievable. For example, Yoder does not describe means for achieving a substantially uniform electron beam. Yoder does not indicate how the detection apparatus can be constructed so as to achieve a linear output from a uniform beam, and in fact, it achieves just the opposite. Thus, Yoder's arrangement is seriously deficient in regard to actual construction of a deflectron having linear response.
Chang, Muray, Lee, MacDonald (see references) have described “microcolumn arrays” of miniature electron guns and elements thereof for the purpose of improved electron beam lithography in semiconductor fabrication, yet they have not explored the potential of employing microcolumn arrays in amplifiers, RF generators or computing.
U.S. Pat. No. 3,922,616 issued to Weiner describes one way to provide gain from an electron beam, by means of an electron bombarded semiconductor. This is commonly called an “EBS” amplifier. The method is based on a p+-i-n+ diode with an intrinsic “i” layer. Kitamura et al (1993, ref 12) explicitly describes an EBS amplifier based on a silicon Schottky diode, but do not employ deflection means. U.S. Pat. No. 4,410,903 issued to Weider describes a heterojunction EBS amplifier based on InGaAs and InP compounds to improve the speed and bandwidth, but these suffer from lack of compatibility with low-cost silicon microfabrication. All three disclosures provide means to improve the gain of an electron beam deflectron amplifier over that of Yoder or Tang et al.
U.S. Pat. No. 5,592,053 issued to Fox et al. describes a variation on the EBS amplifier that provides gain via an electron-beam activated diamond conductor. U.S. Pat. No. 5,355,380 issued to Lin describes a related e-beam excited diamond switch for millimeter wave generation that depends on modulating the current of an electron beam. The principle disadvantage in either is that high beam energies are required with a diamond detector material. This causes extra heating losses, reduced efficiency, and severely limits the deflection gain. Another disadvantage is that Fox does not employ a precision e-beam forming device, such as a microcolumn. Another disadvantage is the difficulty of fabricating high-quality diamond films. Again, beam deflection is not incorporated in the gain mechanism.
A principle disadvantage of following Tang et al., Yoder, or Weiner is that they rely on high current electron beams, which are difficult to focus in low-energy beam systems because of the space charge effect. Lack of focus reduces amplifier gain, decreases bandwidth and increases amplifier distortion. Fox overcomes this with a high energy beam. High current and high energy beams are antithetical to microfabricated electron beam systems. High current and high energy beams dissipate excess anode heating power. High voltage beam circuitry is susceptible to destructive arcing and requires high voltage power supplies, which are difficult to build, bulky and power consumptive, and not amenable to microfabrication.
U.S. Pat. No. 4,328,466 issued to Norris et al describes an EBS amplifier that operates with a sheet beam to disperse the space charge and permit higher beam current, but sheet beams still suffer substantial space charge effects, thereby limiting the beam current and amplifier gain. Norris' amplifier suffers from the complexity of a distributed architecture to achieve high frequency broadband and high power operation, making it unsuitable for low-cost microfabrication.
Low current beams are desirable, yet they reduce amplifier gain. It may be appreciated that there is a need for higher current, but low energy electron beam systems for microfabricated high speed amplifiers.
U.S. Pat. No. 5,041,069 issued to Seiler, U.S. Pat. No. 6,177,909 issued to Reid, and Froberg (ref. 8) have constructed photoconductive antennas which employ semiconductor antenna excitation to generate THz radiation, yet they suffer from uncontrolled wideband transmission, no phase or polarization control, and require complex laser activation with slow pulse repetition rates. As will be seen, the present invention advances the art over all these examples of prior art, simultaneously providing, in different embodiments, controlled wideband modulation, high gain, RF transmission, phase and polarization control.
It will be appreciated in the following description and appended claims that the present invention combines many of the advantages of prior art while overcoming the deficiencies in a novel arrangement, to thereby achieve RF amplifier embodiments possessing higher gain, faster operation, less distortion and lower power consumption. These benefits accrue in almost any RF receiver or transmitter application including wireless networking and antenna beamforming, frequency multiplication, high-speed digital logic and computing.